1. Field of the Invention
The invention relates to an apparatus for receiving digital data transmitted in the form of multiplexed packets in the video channel of a television transmission system. The apparatus includes a demodulator provided with a data clock which can be synchronized to the reception of a bit synchronizing burst. The bit synchronizing burst is a sequence of n bits of alternately 1 and 0 present at the start of each data packet after a line synchronizing signal. The apparatus also includes a demultiplexer to which the data signals and the data clock signals from the demodulator can be transmitted. The apparatus has means for handling the data to be visualized on a screen. The apparatus also has a circuit for recognizing the data for authorizing the transfer of the numerical data to the demultiplexer by means of a recognition signal, if the start of a data packet has been recognized.
2. Related Art
It is known to transmit digital data in the form of multiplexed packets in the video channel of a television transmission system, notably for displaying information on this display screen. Such systems are known under the names of "ANTIOPE" in France, "CEEFAX"in Great Britain and NABTS in USA.
It is known that the transmission of digital data during the normal line scanning period starts with a transmission of a synchronizing burst of bits which are presented in the "ANTIOPE", NABTS and "CEEFAX" systems in the form of a 16-bit word which is identical for these two systems.
By nature, this type of transmission is not bilateral and it is thus necessary to obtain an identification upon reception, ensuring as much as possible that a line of digital data is present instead of an analogue video signal line which could accidentally show a certain resemblance to a start of the transmission of digital data.
For this purpose, it is known to provide a data recognition circuit supplying a recognition signal as early as possible in the course of the transmission corresponding to one television line. The recognition signal authorizes or prevents the transfer of digital data to the demultiplexer depending on whether this line presents the characteristic elements of a bit synchronizing burst at the start of reception.
An arrangement for receiving digital data as described in the opening paragraph is particularly known from the document FR-A-2,496,376.
In the presence of distorted and/or noise signals upon reception, notably when the distance between the transmitter and the receiver is large, the circuits for recognizing data operate with a certain rate of recognition of errors which may involve a rejection of a non-recognized data line or the erroneous recognition of a video signal line as a data line. From this point of view, it is desirable that the circuit for recognizing the data operate with an optimum tolerance to avoid excessive rejection of data lines in the presence of noise. The circuit should nevertheless provide a good safeguard against inadvertent validation of video signal lines.
If an apparatus for receiving digital data is to be realized which is capable of indifferently treating signals transmitted in accordance with the different standards (for example "ANTIOPE", NABTS and "CEEFAX") the circuit for recognizing the data should also be adaptable to the differences in specifications and particularly to the tolerances in the transmission standards of these systems.